The invention relates generally to coverglass materials for spacecraft solar arrays and, in particular to a new coverglass material that provides high UV spectral transmissivity in a robust, flexible, and fully encapsulating format.
Traditional space solar arrays generally consist of solar cells bonded to rigid panel substrates and covered with ceria-doped borosilicate microsheets of glass. The rigid panels below and glass sheet above provide protection for the sensitive photovoltaic cells from the ultraviolet light, atomic oxygen and radiation e.g., protons and electrons, present in the space environment. Although coverglass materials have worked satisfactorily for existing solar cell technologies, their cost, fragility, and lack of complete encapsulation (the metal cell to cell interconnects are not covered by the coverglass) have led many organizations to search for a simpler alternative. In addition, space solar cell developers have indicated that advanced solar cells will require access to a wider spectral range than current coverglass materials provide. These efforts have primarily focused on either silicones or polyimide materials which could be applied in a variety of methods. To date, none have been developed which have the needed spectral transmissivity and environmental stability for the space environment.
A new solar cell design, called inverted metamorphic multijunction (IMM), has been recently developed that offers higher efficiency, plus the potential for significantly lower mass and flexibility. For an IMM cell, 10-15 microns of active semiconductor photovoltaic material are removed from the growth substrate resulting in an extremely efficient solar cell (>32% AM0) which is very light and incredibly fragile. Incorporating this new device into the standard rigid panel substrate and coverglass design negates the mass and flexibility opportunities available with the IMM. Thus, there is an even greater need to find coverglass replacement materials suitable for developing a low mass, environmentally stable, and flexible solar array blanket.
For thin, flexible blankets, environmental protection is required on both sides of the photovoltaic cell in order to shield it from the space environment. Kapton, a polyimide film developed by DuPont, is generally viewed as a reasonable option for a flexible substrate, as it has been used previously and is compatible with the space environment. In order to obtain sufficient thickness for radiation protection, about 8 mils of Kapton would be required to equal the shielding protection of 5 mils of coverglass. Kapton has a coefficient of thermal expansion which is roughly ten times greater than that of the IMM device. Thick Kapton films, such as this, may cause the fragile IMM device to fail under the extreme thermal cycling expected on orbit (<−100° C. to >+100° C.) due to the differences in the coefficient of thermal expansion.
Thus there is a need for a protective, flexible, environmentally stable coverglass replacement for both conventional solar arrays as well as for flexible solar arrays, a coverglass material that can be used with recently developed IMM cells to provide a tremendous increase in mass specific power and volumetric power density.